components/llvm8-base.bst: Filter LLVMTableGen from available libraries.

LLVMTableGen is an internal library that is not published in the
shared library. Importing LLVM through CMake tries to verify existence
of this library and complains it not existing.

Fixes #652.
Status Job ID Name Coverage
  Flatpak
passed #212114522
aarch64
app_aarch64

08:30:37

passed #212114524
armhf
app_arm

03:21:09

passed #212114521
cache_i686 x86_64
app_i686

04:51:30

passed #212114520
cache_x86_64 x86_64
app_x86_64

04:47:54

 
  Vm
manual #212114528
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #212114529
armhf
minimal_systemd_vm_arm

00:03:07

passed #212114526
cache_i686 x86_64
minimal_systemd_vm_i686

00:01:51

passed #212114525
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:01:54